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Time-resolved photoelectron imaging was used to investigate the dynamical evolution of the initially prepared S1 (ππ*) excited state of phenol (hydroxybenzene), catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene), and hydroquinone (1,4-dihydroxybenzene) following excitation at 267 nm. Our analysis was supported by ab initio calculations at the coupled-cluster and CASSCF levels of theory. In all cases, we observe rapid (<1 ps) intramolecular vibrational redistribution on the S1 potential surface. In catechol, the overall S1 state lifetime was observed to be 12.1 ps, which is 1-2 orders of magnitude shorter than in the other three molecules studied. This may be attributed to differences in the H atom tunnelling rate under the barrier formed by a conical intersection between the S1 state and the close lying S2 (πσ*) state, which is dissociative along the O-H stretching coordinate. Further evidence of this S1/S2 interaction is also seen in the time-dependent anisotropy of the photoelectron angular distributions we have observed. Our data analysis was assisted by a matrix inversion method for processing photoelectron images that is significantly faster than most other previously reported approaches and is extremely quick and easy to implement. [ABSTRACT FROM AUTHOR]

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright 1La and 1Lb electronic states of 1ππ* character and spectroscopically dark and dissociative 1πσ* states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited 1La state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived 1Lb state or the 1πσ* state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the 1πσ* state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied. [ABSTRACT FROM AUTHOR]

The authors report time resolved photoelectron spectra of the 1B2(1Σu+) state of CS2 at pump wavelengths in the region of 200 nm. In contrast to previous studies, the authors find that the predissociation dynamics is not well described by a single exponential decay. Biexponential modeling of the authors’ data reveals a rapid decay pathway (τ<50 fs), in addition to a longer lived channel (τ∼350–650 fs) that displays a marked change in apparent lifetime when the polarization of the pump laser is rotated with respect to that of the probe. Since the initially populated 1B2(1Σu+) state may decay to form either S(1D) or S(3P) products (the latter produced via a spin-orbit induced crossing from a singlet to a triplet electronic surface), this lifetime observation may be rationalized in terms of changes in the relative ionization cross section of these singlet and triplet states of CS2 as a function of laser polarization geometry. The experimentally observed lifetime of the longer lived channel is therefore a superposition of these two pathways, both of which decay on very similar time scales. [ABSTRACT FROM AUTHOR]

The authors report time resolved photoelectron spectra of the 1B2(1Σu+) state of CS2 at pump wavelengths in the region of 200 nm. In contrast to previous studies, the authors find that the predissociation dynamics is not well described by a single exponential decay. Biexponential modeling of the authors’ data reveals a rapid decay pathway (τ<50 fs), in addition to a longer lived channel (τ∼350–650 fs) that displays a marked change in apparent lifetime when the polarization of the pump laser is rotated with respect to that of the probe. Since the initially populated 1B2(1Σu+) state may decay to form either S(1D) or S(3P) products (the latter produced via a spin-orbit induced crossing from a singlet to a triplet electronic surface), this lifetime observation may be rationalized in terms of changes in the relative ionization cross section of these singlet and triplet states of CS2 as a function of laser polarization geometry. The experimentally observed lifetime of the longer lived channel is therefore a superposition of these two pathways, both of which decay on very similar time scales. [ABSTRACT FROM AUTHOR]

We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H2 and CO following excitation to the 2141 and 2143 transitions in S1. The CO velocity distributions were obtained using dc slice imaging of single CO rotational states (v=0, jCO=5–45). These high-resolution measurements reveal the correlated internal state distribution in the H2 cofragments. The results show that rotationally hot CO (jCO∼45) is produced in conjunction with vibrationally “cold” H2 fragments (v=0–5): these products are formed through the well-known skewed transition state and described in detail in the accompanying paper. After excitation of formaldehyde above the threshold for the radical channel (H2CO→H+HCO) we also find formation of rotationally cold CO (jCO=5–28) correlated to highly vibrationally excited H2 (v=6–8). These products are formed through a novel mechanism that involves near dissociation followed by intramolecular H abstraction [D. Townsend et al., Science 306, 1158 (2004)], and that avoids the region of the transition state entirely. The dynamics of this “roaming” mechanism are the focus of this paper. The correlations between the vibrational states of H2 and rotational states of CO formed following excitation on the 2143 transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel. [ABSTRACT FROM AUTHOR]

The use of high-resolution ion imaging with complementary state-resolved and 'universal' vacuum ultraviolet probes is presented to address a broad range of fundamental problems in chemical reaction dynamics. Examples include applications in state-correlated unimolecular reactions, ion pair, dissociation dynamics and spectroscopy, crossed-beam reactive scattering, and atomic angular momentum polarization in photodissociation.

The DC slice imaging method is used to record energy and angle-resolved distributions for both ground-state Cl ((super 2)P(sub 3/2) and spin-orbit excited Cl ((super 2)P(sub 1/2)) photofragments produced from the A~-band photolysis of CH3Cl at 193.3 nm. The dissociation is found to be highly impulsive, with ~90 percent of the available energy being released into translation.

Though operations won't begin for several months, Premium Standard Farm's (PSF's) new waste-to-fertilizer plant, being built near Green City, Mo., is clearly the start of something big. The product of [...]

IN MICHAEL STEINBERGER'S article 'Neo-Economics' [March], he states that 'by its very nature, globalization tends to disperse power.' Implicitly in the article, this is purported to describe an erosion of [...]